This invention relates to methods and apparatus for making small-diameter iridium electrodes useful for electrical stimulation of nerves in the central or peripheral nervous systems of animals and humans. Tiny electrodes of this type are especially useful for electrical stimulation of selected regions of the cerebral cortex, and are in some applications arranged in arrays of many (e.g., 10 to as many as 50 or more) closely spaced and centrally supported electrodes.
Iridium is an excellent material for such electrodes due to its stiffness, biocompatibility, low capacitance (as compared to platinum), and acceptance of surface oxidation which prevents long-term erosion after implantation. Small-diameter iridium wire which was formerly commercially available (and believed to have been made by delicate drawing of the wire through dies of decreasing diameter) is no longer produced in diameters smaller than about 125 microns, but the need for smaller diameters continues in the field of nerve-stimulating electrodes.
The techniques of this invention have been used to produce pin-like electrodes with shaft diameters in the general range of 10-to-50 microns, and with tapered or sharpened tips with diameters as small as 1-to-2 microns. These techniques are also useful in controlled blunting of the sharpened tips, polishing of the etched electrodes, and in the application of an insulating coating to regions of the electrode shaft which are not to be in electrical contact with tissue in which the shaft is inserted.